The present invention relates to a new and useful improvement in airplane performance. More specifically, the present invention relates to a system for increasing airplane fuel mileage and decreasing drag suitable for 727 model aircraft, particularly by means of an airplane wing modification kit.
From the first days of commercial air travel, methods and devices for increasing fuel efficiency of commercial aircraft have been desired.
Winglets long have been used in the aircraft industry as a method for reducing drag, the retarding forces which act on an airplane as it moves through the air. Decreased drag results in increased fuel efficiency.
Winglets are small lifting surfaces attached to the outboard end of an airplane wing, commonly at or near to a vertical angle from the wing structure. Winglets function to relocate the tip vortex of an airplane wing further outboard and above the unmodified location. In flight, the substantially inward pointing load carried by the winglets relocates the wing tip vortex.
Due to pressure differentials between wing surfaces at a wing tip, air tends to flow outboard along the lower surface of a wing around the tip and inboard along the wing upper surface. When winglets are added, the relocated wing tip vortex caused by the winglets produces cross-flow at the winglets, which often are perpendicular to the flow across the wing surfaces. The side forces created by such cross-flow contain forward components which reduce drag.
Various winglet configurations and placements have been suggested and are shown in issued patents. Some have been used since the earliest days of manned flight. See, e.g., Arnoux U.S. Pat. No. 1,095,952, patented May 5, 1914, and Hackett U.S. Pat. No. 4,190,219, patented Feb. 26, 1980, which show the use of winglets.
In the 1970's, the National Aeronautics and Space Administration ("NASA") suggested a modern, trapezoidal shape for winglets as a method for reducing drag and increasing fuel efficiency.
NASA also has published a report entitled "KC-135 Winglet Program Review", NASA conference publication 2211. The NASA winglets described in that publication generally are trapezoidal. The leading edge of the winglet is at a 38.degree. angle from the vertical and the trailing edge is at a lesser angle. The tip chord of the NASA winglet is about one-third the length of the root or base chord. The NASA winglet in position is canted outwardly from the wingtip at a cant angle of 15.degree. and positioned toward the trailing edge of the wing.
Airframe manufacturers also have proposed the use of winglets. Ishimitsu U.S. Pat. Nos. 4,205,810, patented Jun. 3, 1980 and 4,245,804, patented Jan. 20, 1981, disclose winglets. The earlier Ishimitsu '810 patent shows and describes winglets which are vertically positioned with respect to the wings and are not canted outwardly. The later Ishimitsu '804 patent shows and describes winglets which are canted outward, enabling greater control of the winglet center of gravity to alleviate wing flutter, or oscillation. A strake, or band, having an aerodynamically contoured leading edge is added between the leading edge of the winglet and the upper surface of the wing.
Performance improvements obtained as a result of winglet additions are reduced drag and increased wing buffet margin. Wing buffet occurs when lift on a wing is so great that it causes flow separation and wing stall, and subsequently limits the boundary for the aircraft. The reduction in drag brings increased aircraft fuel mileage at all altitudes, while the increased buffet margin allows the aircraft to fly at higher altitudes where fuel mileage is increased.
While winglets have been shown to increase fuel mileage in 727 model aircraft, the addition of winglets also creates a major operational problem. Inherent in the way that a winglet functions to reduce drag is the tendency of the winglet to increase wing bending and shear loads over the outboard portion of the wing. Such loads are further increased when a winglet is canted outward from the vertical.
Winglets could have no beneficial effect without also inducing this load increase. However, as the wing structure of 727 model aircraft originally was designed only to carry the load of an unmodified wing, it is a difficult problem to sustain the higher load introduced by the addition of a winglet while simultaneously maintaining margins of safety that satisfy Federal Aviation Administration regulations. The excessive wing loads and stress caused by the addition of winglets has made them impractical for use in 727 model aircrafts. The alternative, reinforcement of existing wing structure to support the winglets, is extremely expensive.